1. Field of the Invention
This invention relates to quantum interference devices for converting signals carried by magnetic fields, electric fields, light (intensity, wavelength) or the like into electric signals by use of the Aharonov-Bohm effect (AB effect), optical Stark effect and so forth, and methods for processing electron waves utilizing a real space transfer.
2. Related Background Art
Conventionally, there has been developed a quantum interference device as shown in FIG. 1. This device operates in the following fashion.
An electron wave injected by an electrode 10 is propagated through a part (left end part in FIG. 1) in which the distance between two electron waveguides 11 and 12 is made short enough to couple the electron wave, and arrives at a decoupling part 15 or wave branching part. In this decoupling part 15 which is indicated by dotted loop lines on the left side in FIG. 1, the distance between two electron waveguides 11 and 12 is made longer than that in the coupling part at the left end part, so that the electron wave divides into two waves in this decoupling part. In the coupling part 16 Si is doped so that the Fermi level in the two electron wave waveguides 11 and 12 may be located between first and second quantum levels or subbands. Thus, the electron wave is concentrated solely into the first level below the Fermi level, leading to the coupling of the electron waves. Practically, the coupling and decoupling are achieved by varying the thickness of an AlAs layer which is a barrier layer 13. That is, the barrier 13 is thinner at the two ends so that there is considerable tunneling between the waveguides or wells 11 and 12 at the ends but hardly any tunneling in the central region.
In the decoupling part 15, the phase difference between the two electron waves is due to the AB effect (more in detail, magnetostatic AB effect) by applying thereto the magnetic field in a direction normal to the sheet of FIG. 1. The two electron waves are coupled at the coupling part 16 on the right side in FIG. 1 indicated by dotted loop lines, and thus there is generated a bonding state having a lower energy or an antibonding state having a higher energy. Here, the Fermi level is adjusted by doping, so that only the electron wave having lower energy reaches a drain 14. That is, an effect similar to the optical interference effect occurs between the two coupling electron waves, and there are cases where electrons can reach the electrode 14 at the detection side and where electrons do not reach the electrode. As a result, on/off control of the device is performed.
The length of the decoupling part should be less than 1 .mu.m at liquid helium (LHe) temperatures under present day technologies, because coherency of the electron waves needs to be maintained.
This device is disclosed in S. Datta, et., Appl. Phys. Lett. 48(7), Feb. 17, 1986, pp. 487-489.
Further, there has also been proposed a device such as shown in FIG. 2, which utilizes real space transfer without using the tunneling phenomenon. The device of FIG. 2 does not utilize the electron wave, but such a device can readily be modified to take the electron wave out of the quantum structure by replacing a layer 20 through which current flows with a quantum well or the like.
The idea for this device is based on the energy discontinuity at the heterjunction region (junction between layers 20 and 21) which differs between .GAMMA. and X valleys in a wave number space or k space. For example, if the Al mole fractions x,y of the layers 20 and 21 are assumed to be 0.33 and 0.6, respectively, the energy discontinuities at the junction between the n-AlGaAs layer 20 and i-AlGaAs layer 21 are +0.23 eV at the .GAMMA. valley and +0.03.about.0.075 eV at the X valley. Here, where the energy of the i-AlGaAs layer 21 is higher than that of the n-AlGaAs layer 20 as a reference, the sign is +, and where lower, the sign is -. So, while electrons at the .GAMMA. valley cannot easily come out of the n-AlGaAs layer 20 owing to the energy barrier by the layer 21, electrons at the X valley can readily travel out of the n-AlGaAs layer 20. As a result, it can be seen that desired operations such as on/off control of the current will be achieved by transferring the electrons in the n-AlGaAs layer 20 from the .GAMMA. valley to X valley.
In order to obtain this operation, a voltage (V.sub.s) is applied along the layer surface of the device (lateral direction in FIG. 2) so that the wave number k of the electron may be increased to cause the above electron transfer. At the same time, a voltage (V.sub.T) is applied also in the direction of layer thickness so as to cause the current to flow out in this direction (perpendicular direction in FIG. 2). When the voltage Vs is switched between V.sub.s =1 V (assuming that the distance between electrodes is 1 .mu.m) and V.sub.s =0 V at a temperature of 77 K, the ratio of current flowing in the perpendicular direction of layer thickness due to the transfer to the X valley is 10.sup.5 (V.sub.s =1 V):1 (V.sub.s =0 V). Thus, electrons can efficiently transfer from the .GAMMA. valley to X valley by the application of a voltage in the lateral direction of layer surface. The above structure is disclosed in the Jasprit Singh, Appl. Phys. Lett. 55(25 ), Dec. 18, 1989 pp. 2652-2654.
However, those prior art devices have the following disadvantages. In the device of FIG. 1, there are problems in that the electron wave in the antibonding state reflected by the coupling part at the side of the drain 14 generates heat in a region between the source 10 and drain 14 and in that the reflected electron wave passes the source 10 as noise to other devices. Further, since only such is measured in which the phase difference between two electron waves is 2 m.pi. (m:integer), the S/N of a signal will be poor.
On the other hand, in the device of FIG. 2 in which the voltage is applied in the lateral direction of layer surface as a means for attaining the real space transfer, there is a problem that such a device is unsuitable where it is required that the expansion or dispersion of wave numbers of travelling electrons be as small as possible to achieve sharper interference, such as quantum interference devices. Further, the problem of S/N also exists in this device.